Process for isolation of pure substances from mixtures of high-b

Details Process for isolation of pure substances from mixtures of high-b Process for isolation of pure substances from mixtures of high-b

1998-01-08

2000-08-29

McKane, Joseph

Organic compounds -- part of the class 532-570 series

Organic compounds

Heterocyclic carbon compounds containing a hetero ring...

C07D31172

Patent

active

061111171

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a process for isolating pure substances from mixtures of high-boiling air- and/or temperature-sensitive substances which require a high separation efficiency by rectification under medium vacuum, in particular a process for the rectification of crude vitamin E acetate, for the purpose of purifying from lower-boiling and higher-boiling impurities, and to columns suitable for this process.
The workup of product mixtures by distillation generally affords the best results in the case of countercurrent distillation (also called retification), ie. a specific distillation process with a countercurrent of reflux flowing downwards and vapor flowing upwards in rectification columns. The rectification columns normally used for this purpose are such that the mixture to be separated is introduced into the middle part of the column, and the vapor on its way through the column from the bottom to the top becomes enriched in more volatile components while the reflux from top to bottom becomes enriched in less volatile components. Mass and heat transport is intensified by elements fitted inside the column, such as column plates or packings, which ensure a sufficient contact time of the phases and a sufficiently large phase boundary area. However, these column internals, together with the reflux flowing downwards, result in a resistance in the column which is called the pressure drop. The pressure drop in a column depends not only on the nature and amount of the compounds to be rectified but also very strongly on the nature of the column internals.
Generally used for the fractionation of high-boiling substance mixtures which require a high separation efficiency are rectification columns which have packings which are built up systematically in regular geometry and have defined areas for the countercurrent phases to pass through, because packings with a regular structure are distinguished in comparison with random packings by the possibility of higher flow rates and a better separation effect, and have a lower specific pressure drop and a smaller required packing volume and thus also a smaller necessary mass and heat exchanging height. They are therefore used in all vacuum rectifications in which, because of the temperature-sensitivity of the mixture to be separated, it is particularly important to limit the pressure drop in the column.
Particularly suitable column packings are metal cloth packings of the BX and CY types supplied by Sulzer (cf. Sulzer company publication "Trennkolonnen fur Destillation und Absorption") and metal cloth packings with a similar effect supplied by other companies such as Montz GmbH.
A diagrammatic representation of such columns is to be found, for example, on page 103 of the textbook "Thermische Trennverfahren" by Klaus Sattler, VCH Verlagsges.mbH, Weinheim (FRG), 1988. Concerning further details of the rectification of substance mixtures, we refer to this textbook by Klaus Sattler, pages 101-225, in particular 120-160 and 199-214.
The highest product temperature occurs in the bottom of a column. Apart from the overhead pressure, it is closely determined by the pressure drop from the column internals derived from the required separation efficiency. In rectification columns, the bottom temperature is not significantly affected by reducing the overhead pressure to less than 0.5 mbar.
The thermal stressability of many high-boiling mixtures is so low that, despite the use of the described metal cloth packings with ordered structure and overhead pressures in the column of only 0.5 to 1 mbar, the pressure drop at the cloth packings which are required for the necessary separation efficiency would result in bottom temperatures which are above the decomposition range for the compounds to be separated. This is why to date the fractionation of such mixtures by distillation has generally been carried out in the high vacuum range (about 10.sup.-1 to 10.sup.-5 mbar), ie. short-path distillations or molecular distillations are used. However, in the case of mixtures with low relative vola

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